1. Field of the Invention
The present invention relates to a semiconductor optical device, such as a semiconductor laser device or the like, and a manufacturing method of the semiconductor optical device.
2. Description of the Related Art
With a recent explosive increase of individuals using the Internet, rapid increase of an information transmission speed and enlargement of transmission capacity are required, and optical communication is expected to remain bearing an important role. As a light source for use in optical communication, a semiconductor laser device is mainly used. For transmission in a short distance of about 10 km, a direct modulation system is used to directly drive a semiconductor laser, using an electric signal. According to this system, a module can be implemented in a simple structure and thus requires smaller power consumption as well as a fewer number of components, which can reduce costs. Meanwhile, for optical transmission in a long distance in excess of 10 km, as direct modulation of a semiconductor laser alone cannot achieve such transmission, an electro-absorption modulator integrated semiconductor laser device having an optical modulator unit integrated is used.
To enlarge transmission capacity of optical communication, it is necessary to achieve a higher modulation speed of a semiconductor laser than the current one. However, as the modulation speed of a semiconductor laser is subjected to restriction due to a product of a device capacitance and a device resistance (a CR time constant), further increase of the modulation speed requires reduction of device resistance or device capacitance.
There are roughly two types available for a basic structure of a semiconductor optical device, namely, a buried-hetero (BH) structure and a ridge wave-guide (RWG) structure. Generally, a semiconductor optical device has an active layer, such as a multiple-quantum-well (MQW) layer or the like, for emitting light through recombination of an electron and a hole, deposited between a p-type cladding layer and an n-type cladding layer. Further, in order to achieve a lasing spectrum in a single mode, a diffraction grating layer is formed within a cladding layer.
To reduce device resistance, it is effective to reduce resistance of a p-type cladding layer having a hole, which has lower mobility compared to an electron, as a carrier. In the above, conventionally, zinc (Zn) is used as a dopant for a p-type cladding layer. However, as Zn has readily diffusible nature, as known, excessive increase of the doping concentration of a p-InP cladding layer in order to reduce device resistance results in remarkable increase of the amount of Zn diffusion in an adjacent MQW layer. This increases optical loss, and thus deteriorates the characteristic of a semiconductor laser.
In a BH structure device, in particular, as Zn is excessively diffused in an insulating portion around the MQW layer and insulation is thereby deteriorated. As a result, a leak path of current is formed, which increases current components not being introduced into the MQW layer but flowing avoiding the MQW layer. Therefore, considering Zn diffusion, there is a limit to the amount of Zn doping concentration that can be increased, and thus to the amount of device resistance that can be reduced by increasing the Zn doping concentration.
IEEE Journal of Quantum Electronics, Vol. 40, No. 12, December 2004 reports Mg as a new dopant that is substituted for Zn. That is, it is reported that use of Mg for a p-type cladding layer made of AlGaInP enables doping with lower diffusion at higher concentration than that when Zn is used.
In connection with a problem of doping delay caused when Mg is used as a p-type dopant for AlGaInP material, JP H06-13334 A discloses use of mixed gas of Mg organic metal compound and Al organic metal compound as p-type impurity in MOVPE crystal growth.